1. Field of the Invention
The present invention is related to a fast frequency locking method and architecture realized by employing adaptive asymmetric charge-pump current mechanism, especially to a method which can enable fast locking of circuit frequency, furthermore to a method that applying an existing current source within a charge-pump to charge/discharge filters of circuit and simultaneously applying an assistant current source at a side of the existing current source to provide a large output current while frequency change, so that frequency locking can be achieved within a shorter period of time.
2. Description of the Prior Arts
Nowadays, accompanying the improvement of circuit design and manufacture technique, processing speed of electronic circuit increases as well. Therefore, a phase-locked circuit with fast locking function and has a short locking and recovery period is needed. While designing the fast locking phase-locked circuit, a dilemma of fast locking and low noise must be considered. That is, when frequency locking process of phase-locked loop becomes faster, its filter's bandwidth is increased such that noises generated within circuit are also increased relatively; on the other hand, if bandwidth of filter is narrowed to reduce noise so as to solve the above drawback, then processing speed of phase-locked loop becomes comparatively slow accordingly.
Please refer to FIG. 1, a traditional charge-pump circuit is illustrated whose circuit is composed of elements such as: a pair of frequency-dependent main current sources, a voltage controlled oscillator, an impedance, a sampling frequency, a fixed frequency and a phase detector, etc. Wherein, a charging current source is provided and is equal to discharging current source, so as to enable control voltage of voltage controlled oscillator (VCO) to provide an efficient discharging conduit while control voltage is too high and output frequency is too fast. In this way, control voltage can be decreased and frequency can be reduced. Hence, during a frequency locking process of the whole circuit, a transient state of locking process is generated that the frequency will oscillate for a period of time till a stable value is reached. This is the operating principle of traditional charge-pump circuit of the prior arts. Assuming the range of frequency change is unknown, to achieve a best frequency locking state, charging current and discharging current shall be equal in size so that frequency won't rise too fast and descend too slow. However, in this traditional frequency locking structure, some leakage current is still being generated within circuit during frequency locking process. The leakage current also generates interference noises toward the circuit that consequently causes prolong of frequency locking process. The present invention can overcome this drawback of the prior arts.